Optical structures that scatter or diffuse light generally function in one of two ways: (a) as a surface diffuser utilizing surface roughness to refract or scatter light in a number of directions; or (b) as a bulk diffuser having flat surfaces and embedded light-scattering elements.
A diffuser of the former kind is normally utilized with its rough surface exposed to air, affording the largest possible difference in index of refraction between the material of the diffuser and the surrounding medium and, consequently, the largest angular spread for incident light. However, some prior art light diffusers of this type suffer from two major drawbacks: a high degree of backscattering and the need for air contact. Backscattering causes reflection of a significant portion of the light back to the originating source when it should properly pass through the diffuser, lowering the efficiency of the optical system. The second drawback, the requirement that the rough surface must be in contact with air to operate properly, may also result in lower efficiency. If the input and output surfaces of the diffuser are both embedded inside another material, such as an adhesive for example, the light-dispersing ability of the diffuser may be reduced to an undesirable level.
In one version of the second type of diffuser, the bulk diffuser, small particles or spheres of a second refractive index are embedded within the primary material of the diffuser. In another version of the bulk diffuser, the refractive index of the material of the diffuser varies across the diffuser body, thus causing light passing through the material to be refracted or scattered at different points. Bulk diffusers also present some practical problems. If a high angular output distribution is sought, the diffuser will be generally thicker than a surface diffuser having the same optical scattering power. If however the bulk diffuser is made thin, a desirable property for most applications, the scattering ability of the diffuser may be too low.
Despite the foregoing difficulties, there are applications where an embedded diffuser may be desirable, where the first type of diffuser would not be appropriate. For example, a diffuser layer could be embedded between the output polarizer layer and an outer hardcoat layer of a liquid crystal display system to protects the diffuser from damage. Additionally, a diffuser having a thin profile, which will retain wide optical scattering power when embedded in other materials and have low optical backscatter and therefore higher optical efficiencies than conventional diffusers, would be highly desirable.
U.S. Pat. No. 6,093,521 describes a photographic member comprising at least one photosensitive silver halide layer on the top of said member and at least one photosensitive silver halide layer on the bottom of said member, a polymer sheet comprising at least one layer of voided polyester polymer and at least one layer comprising nonvoided polyester polymer, wherein the imaging member has a percent transmission of between 38 and 42%. While the voided layer described in U.S. Pat. No. 6,093,521 does diffuse back illumination utilized in prior art light boxes used to illuminate static images, the percent transmission between 38 and 42% would not allow a enough light to reach an observers eye for a liquid crystal display. Typically, for liquid crystal display devices, back light diffusers must be capable of transmitting at least 65% and preferably at least 80% of the light incident on the diffuser.
In U.S. Pat. No. 6,030,756 (Bourdelais et al), a photographic element comprises a transparent polymer sheet, at least one layer of biaxially oriented polyolefin sheet and at least one image layer, wherein the polymer sheet has a stiffness of between 20 and 100 millinewtons, the biaxially oriented polyolefin sheet has a spectral transmission between 35% and 90%, and the biaxially oriented polyolefin sheet has a reflection density less than 65%. While the photographic element in U.S. Pat. No. 6,030,756 does separate the front silver halide from the back silver halide image, the voided polyolefin layer would diffuse too much light creating a dark liquid crystal display image. Further, the addition of white pigment to the sheet causes unacceptable scattering of the back light.
In U.S. Pat. No. 4,912,333, X-ray intensifying screens utilize microvoided polymer layers to create reflective lenslets for improvements in imaging speed and sharpness. While the materials disclosed in U.S. Pat. No. 4,912,333 are transmissive for X-ray energy, the materials have a very low visible light energy transmission which is unacceptable for LC devices.
In U.S. Pat. No. 6,177,153, oriented polymer film containing pores for expanding the viewing angle of light in a liquid crystal device is disclosed. The pores in U.S. Pat. No. 6,177,153 are created by stress fracturing solvent cast polymers during a secondary orientation. The aspect ratio of these materials, while shaping incident light, expanding the viewing angle, do not provide uniform diffusion of light and would cause uneven lighting of a liquid crystal formed image. Further, the disclosed method for creating voids results in void size and void distribution that would not allow for optimization of light diffusion and light transmission. In example 1 of this patent, the reported 90% transmission includes wavelengths between 400 and 1500 nm integrating the visible and invisible wavelengths, but the transmission at 500 nm is less that 30% of the incident light. Such values are unacceptable for any diffusion film useful for image display, such as a liquid crystal display.
The need for having a thinner and stiffer base for imaging products is well recognized. In addition to providing cost advantage, thinner supports can fulfill many other criteria. For example, in motion picture and related entertainment industry, thinner photographic base allows for longer film footage for the same sized reels. However, a reduction in thickness of the base typically results in a reduction in stiffness, which can have detrimental effects in terms of curl, transport, and durability. For electronic display materials, such as liquid crystal display, it is desirable that the components be light in weight and flexible.
Recently, nanocomposite materials prepared using smectite clays have received considerable interest from industrial sectors, such as the automotive industry and the packaging industry, for their unique physical properties. These properties include improved heat distortion characteristics, barrier properties, and mechanical properties. The related prior art is illustrated in U.S. Pat. Nos. 4,739,007; 4,810,734; 4,894,411; 5,102,948; 5,164,440; 5,164,460; 5,248,720, 5,854,326, 6,034,163. However, the use of these nanocomposites as thinner and stiffer display components with specific optical properties has not been recognized.
In order to obtain stiffer polymeric supports using smectite clays, the clays need to be intercalated or exfoliated in the polymer matrix. There has been a considerable effort put towards developing methods to intercalate the smectite clays and then compatibilize with thermoplastic polymer matrix. This is because the clay lattice is naturally hydrophilic, and it must be chemically modified to be organophilic in order to allow its incorporation in the polymer matrices. To obtain the desired polymer property enhancements, most intercalation techniques developed so far are batch processes, time consuming and lead to increasing the overall product cost.
There are two major intercalation approaches that are generally used—intercalation of a suitable monomer followed by polymerization (known as in-situ polymerization, see A. Okada et. Al., Polym Prep. Vol. 28, 447, 1987) or monomer/polymer intercalation from solution. Polyvinyl alcohol (PVA), polyvinylpyrrolidone (PVP) and polyethylene oxide (PEO) have been used to intercalate the clay platelets with marginal success. As described by Levy et. al, in “Interlayer adsorption of polyvinylpyrrolidone on montmorillonite”, Journal of Colloid and Interface Science, Vol 50 (3), 442, 1975, attempts were made to sorb PVP between the monoionic montmorillonite clay platelets by successive washes with absolute ethanol, and then attempting to sorb the PVP by contacting it with 1% PVP/ethanol/water solutions, with varying amounts of water. Only the Na-montmorillonite expanded beyond 20 Å basal spacing, after contacting with PVP/ethanol/water solution. The work by Greenland, “Adsorption of polyvinyl alcohol by montmorrilonite”, Journal of Colloid Science, Vol. 18, 647-664 (1963) discloses that sorption of PVA on the montmorrilonite was dependent on the concentration of PVA in the solution. It was found that sorption was effective only at polymer concentrations of the order of 1% by weight of the polymer. No further effort was made towards commercialization since it would be limited by the drying of the dilute intercalated layered materials. In a recent work by Richard Vaia et.al., “New Polymer Electrolyte Nanocomposites: Melt intercalation of polyethyleneoxide in mica type silicates”, Adv. Materials, 7(2), 154-156, 1995, PEO was intercalated into Na-montmorillonite and Li-montmorillonite by heating to 80° C. for 2-6 hours to achieve a d-spacing of 17.7 Å. The extent of intercalation observed was identical to that obtained from solution (V. Mehrotra, E. P. Giannelis, Solid State Commun., 77, 155, 1991). Other, recent work (U.S. Pat. No. 5,804,613) has dealt with sorption of monomeric organic compounds having at least one carbonyl functionality selected from a group consisting of carboxylic acids and salts thereof, polycarboxylic acids and salts thereof, aldehydes, ketones and mixtures thereof. Similarly U.S. Pat. No. 5,880,197 discusses the use of an intercalant monomer that contains an amine or amide functionality or mixtures thereof. In both these patents and other patents issued to the same group the intercalation is performed at very dilute clay concentrations in an intercalant carrier like water. This leads to a necessary and costly drying step, prior to intercalates being dispersed in a polymer. Disclosed in WO 93/04118 is the intercalation process based on adsorption of a silane coupling agent or an onium cation such as a quaternary ammonium compound having a reactive group that is compatible with the matrix polymer.
There are difficulties in intercalating and dispersing smectite clays in thermoplastic polymers. This invention provides a technique to overcome this problem. It also provides an article with improved dispersion of smectite clays that can be incorporated in a web.